An Efficient Full Long-Term Approach for Mooring Systems Extreme Response Estimation Using a Multi-Dimensional Joint Environmental Model

In the long-term scenario, the environmental actions to which floating offshore structures are subjected to, such as waves, wind and current, are non-stationary processes. However, this long-term behavior is usually modeled as a series of short-term stationary conditions. In a full long-term analysis approach (FLTA), an estimate of the N-year response can be obtained through an integration of short-term peaks probability distributions over all these short-term environmental conditions. This paper applies an innovative and more efficient full long-term integration method based on the Importance Sampling Monte Carlo Simulation (ISMCS) method, where the uniform distribution is used as the sampling function. In parallel, a multi-dimensional joint environmental model that statistically describes all relevant environmental parameters is developed, contemplating linear and directional variables, and treating wind sea and swell waves individually. The methodology is applied for two FPSO systems installed in Brazilian deep waters. It is shown through a Bootstrap procedure that ISMCS is a numerically stable procedure and provides reliable estimates for the long-term N-year response with a feasible number of required numerical simulations. It is also shown that it can be a powerful tool to account for the simultaneous occurrence of wind sea and swell waves in structural response evaluations and that its use can lead to more accurate extreme tension response estimates, without being overly conservative.